Q. What is the difference between Manual, Open Loop and Closed Loop control?

A. Manual Control
Tension is controlled manually by operator adjustment. 

Similar to: While driving a car and ascending a hill, the operator attempts to adjust the accelerator by visually estimating the incline. (No speedometer)

OpenLoop control

Tension is controlled by a control based upon calculations made from a sensor input or follower arm input. However, there is no direct measurement of actual tension.

Similar to: A car is equipped with a sensor that detects changes in incline and a controller that makes an accelerator adjustment to try to keep speed constant. (Sensor is not directly measuring speed.

Closed Loop Control

Tension is controlled by the Controller, which corrects for errors between the tension set point and the sensor feedback signal. Feedback can be provided by load cells or a dancer arm sensor.

Similar to: Controlling the speed of a car by adjusting the accelerator based upon the speedometer reading. (Actual speed feedback or "Cruise Control")

Q. Do I need a dancer Closed Loop system or load cell based Closed Loop system for my application?

A. Dancer systems are typically used when accumulation is needed to allow for tension variations created by out-of-round rolls, indexing, or start / stop applications. In these cases or any others where relatively large fluctuations of web velocity are expected to occur, a dancer system will absorb the large transients and maintain the desired tension.

Load cell based systems are typically used for precise control of tension in a continuous moving web or for round rolls.

Q. Why does my dancer arm become unstable near core?

A. Dancer systems apply a certain magnitude of change in torque for a given magnitude of change in dancer arm position. The same magnitude of torque output change at core results in a much larger tension than the same output change at full roll, because it is applied over a much shorter torque arm. The result is that the arm may overshoot its setpoint and the controller will overcorrect repeatedly, creating an unstable condition. Typically, this situation can be eliminated with careful tuning of the controller, as well as setting the control gain proportionate to the roll size. The VERSATEC, a gain compensating dancer control, can serve this function.

Q. What are the advantages of magnetic particle devices?

A. MAGPOWR magnetic particle brakes and clutches are infinitely adjustable while the machine is running because the coil current is easily controllable. This current provides repeatable torque that is unaffected by speed or temperature. Silent operation, along with smooth stops and starts, are features inherent in magnetic particle devices. MAGPOWR magnetic particle brakes and clutches will provide many years of trouble-free service. 

Q. Why do I need to install a run/stop switch when installing a MAGPOWR control?

A. When the process stops, a closed loop control will continue to attempt to correct for error, but without a moving web, it cannot. A switch is required to utilize the soft start and stop features for the closed loop controls that are so equipped. The ìRUNî mode for all MAGPOWR controls is a normally open circuit, so it must be connected to ìCOMMONî to activate the ìSTOPî mode. Typically this is accomplished with a switch for normal machine operation, but some controls will at least require a jumper wire to enter certain programming or diagnostic modes.

Q. What is the life expectancy of my magnetic particle device?

A. The life expectancy of a MAGPOWR magnetic particle device is application dependent. The unit must be sized according to the parameters of the application to ensure that the heat dissipation and torque requirements are within the acceptable range of the magnetic particle device. Properly sized units will give many years of trouble-free operation.

Q. Why should I use torque control vs. speed control?

A. Torque control is a direct, easy way to control tension. For torque control, tension is determined by two factors, torque and roll radius. When operating in speed control mode, the elasticity of the material, tension in the prior zone, velocity of the nips, rewind, and web play an important factor in how well tension will be controlled. Therefore, with fewer variables and a direct proportional relationship determining the actual tension in the material, torque control is a more accurate way to control tension.

Q. What is the advantage of a current regulated power supply?

A. As a magnetic particle clutch or brake heats up during normal operation, the resistance of the copper in the coil can increase by 30%. Because clutch or brake torque is proportional to current, a constant current is required to provide constant torque. If only the voltage is regulated, the current will drop as the coil resistance increases; therefore, using a voltage regulated power supply will result in unwanted changes in web tension. Using a current regulated amplifier to control a magnetic particle clutch or brake will provide consistent torque output and more precise control of tension.

Q. What is reverse current?

A. Reverse current is a feature on all MAGPOWR power supplies intended to minimize drag torque. By adding a small amount of reverse current when the control signal is reduced to zero, any residual magnetism that remains in the iron parts will be negated. This provides the widest possible torque range with any magnetic particle device.